In traditional NTSC color television receivers, the television viewer controls the hue of the color by phase shifting the subcarrier reference signals applied to chroma demodulators. This results in a rotation of the “phase plot” of the chroma signal with an attendant hue shift of the demodulated color difference signals. This is illustrated, for example, in FIG. 1 showing the results from rotation of a “phase plot” of a chroma signal with the attendant hue shift of the demodulated color difference signals in the context of a traditional NTSC color television receiver. Several representative colors, namely, red, blue and green, are illustrated in a polar plot. It should be noted that the chroma amplitude represented by the length of the color vectors is substantially unaltered by the hue shift.
The foregoing worked well in the context of traditional NTSC color television receivers, but with advances towards IDTV, EDTV and HDTV as well as evolutions in PAL and SECAM towards customer controls like NTSC products, it becomes desirable to accomplish color (i.e. saturation) and tint (i.e., hue) controls on the base band color difference signals themselves.
The prior art includes a number of efforts to adjust color hue and saturation in a variety of different contexts. One example is described in U.S. Pat. No. 4,788,586 issued on Nov. 29, 1988 to Robert R. Eckenbrecht and entitled “Controller for Adjusting Color Hue and Saturation of Images Generated from Signals in a Non-Broadcasting Video System.” Eckenbrecht describes a color tint controller for a closed circuit video system which receives red and blue color difference signals and selectively changes the amplitude of those signals to adjust the color of a displayed image. Other U.S. patents that may be relevant to the adjustment of hue and color saturation include U.S. Pat. Nos. 4,528,586; 4,554,576 and 4,562,460.
Controlling tint by processing color difference signals may cause a problem. Specifically, processing color difference signals for providing tint control over a wide range of hue shift angles may result in undesirable color amplitude variations.